Targeting Vulkan through Rusticl/Zink

The mesa homebrew package links against an unsupported version of LLVM.
This makes it unusable to run OpenCL.jl on Macs.

This can be achieved by building a JLL for:
- Mesa (Zink + Rusticl)
- MoltenVK

Some points:
- We need a minimal build of Mesa with only Rusticl and Zink supported. I'm not sure if llvm is a required dependency here.
- MoltenVK doesn't seem to have a direct dependency on LLVM.

Building Mesa (Zink + Rusticl) also has the advantage to support many more GPUs than Apple like Qualcomm, Mediatek, Rockchip etc (anything with good enough Vulkan support)